The present invention relates to a liquid crystal display device comprising a planar lighting device and a liquid crystal display panel, the former comprising a light guide plate that emits illuminating light through a light exit plane by diffusing light emitted by a light source.
There is known a liquid crystal display device comprising a liquid crystal display panel and a backlight unit or a planar lighting device for illuminating the liquid crystal display panel by radiating light from behind the liquid crystal display panel. The backlight unit is configured using a light guide plate for irradiating the liquid crystal display panel by diffusing light emitted by an illuminating light source and optical members such as a prism sheet and a diffusion sheet.
Backlight units currently used in liquid crystal display devices such as large liquid crystal televisions are predominantly of a type called direct illumination type having no light guide plate but comprising optical members such as a diffusion plate disposed immediately above the illuminating light source. This type of backlight unit comprises a light source constituted by cold cathode tubes provided on the rear side of the liquid crystal display panel whereas white reflection surfaces inside the backlight unit secure uniform light amount distribution and a necessary brightness. To achieve a uniform light amount distribution with the direct illumination type backlight unit, however, the backlight unit needs to have a given thickness, say about 30 mm, in a direction perpendicular to the liquid crystal display panel. While demands of still thinner backlight units are expected to grow in the future, achieving a further reduction of thickness to say 10 mm or less with a direct illumination type backlight unit is deemed difficult in view of uneven light amount distribution expected to accompany that type.
Thus, there has been proposed a backlight unit of a type using a light guide plate that is formed of a transparent resin containing scattering particles for diffusing light (see JP 07-36037 A, for example).
JP 07-36037 A, for example, discloses a light diffusion light guide light source device comprising a light diffusion light guide member having at least one light entrance plane region and at least one light extraction plane region and light source means for admitting light through the light entrance plane region, the light diffusion light guide member having a region that has a tendency to decrease in thickness with the increasing distance from the light entrance plane.
In the planar lighting device mentioned above, light emitted by the light source and admitted through the light entrance plane into the light diffusion light guide member receives a single or a multiple scattering effect at a given rate as the light propagates through the inside of the light diffusion light guide member. Moreover, a significant proportion of light that reaches both end planes of the light diffusion light guide member or a surface of a reflector is reflected and returned back into the diffusion light guide member.
The above composite process produces light beam that is emitted through the light exit plane highly efficiently with a directivity to travel obliquely forward as viewed from the light source. Briefly, light radiated by the light source is emitted through the light extraction plane of the light diffusion light guide member.
Thus, the prior art literature mentioned above purportedly states that a light guide plate containing scattering particles mixed therein is capable of emitting uniform light with a high light emission efficiency.
As regards the light guide plate used in the planar lighting device, there have been disclosed a light guide plate in the form of a flat plate and a light guide plate composed of a portion shaped to have a region with a tendency to grow thinner with the increasing distance from the light entrance plane attached to the other portion, as well as the light guide plate mentioned above that is shaped to have a region with a tendency to grow thinner with the increasing distance from the light entrance plane.
Resides fluorescent tubes such as cold cathode tubes, light emitting diodes (hereinafter referred to also as “LEDs”) may also be used to provide a light source for supplying light into the light guide plate of the planar lighting devices as mentioned above (see JP 2005-183139 A and 2006-47975 A).
Use of LEDs, which are capable of emitting light having a high directivity, to provide a light source enables light admitted into the light guide plate to be guided deeper into the light guide plate and hence a larger planar lighting device to be designed. Another advantage is that LEDs help achieve a simple configuration of the power supply.
JP 2005-183139 A discloses a liquid crystal display device comprising color compensation means that performs color compensation through subtractive color mixing. The color compensation means is located somewhere between a white light source and a display surface of the liquid crystal display element, say either between the light entrance plane of the light guide plate and the light source, or between the radiating plane of the light guide plate and the liquid crystal display element, or in both of these locations. Note that the white light source used therein is of a type to produce white light through additive color mixing using a first light source for emitting monochromatic blue light and a second light source for emitting monochromatic red light in such a manner as to blend blue light emitted by the first light source, green light produced from the monochromatic blue light through wavelength conversion, and red light emitted by the second light source.
JP 2006-47975A discloses a color image display device comprising light shutters, a color filter having at least three component colors of red, green, and blue corresponding to the light shutters, and a backlight for transmitted lighting. The backlight has LEDs provided therein. Now, let any wavelength occurring at 5 nm increments in a visible range of 380 nm to 780 nm be λn nm, the spectral transmittance [%] of a red pixel of the color filter at the wavelength λn nm be TR(λn), and the relative luminescence intensity normalized by the entire luminescence intensity at the wavelength λn nm of light emitted by the backlight be I(λn). Then I(620−680)×TR(620−680)≧1.1 holds in the above color image display device.
JP 07-253577 A discloses a color display device comprising light shutters for controlling transmitted light amount, color filters having three component colors of red, green and blue, and a light source. The light source is a single monochromatic white light source having therein synthesized monochromatic luminescence properties of the three colors and produces light having peak wavelengths that coincide with the peak wavelengths of the respective component colors in the transmitted wavelength ranges and having a spectroscopic property characterized by relatively narrow ranges. The three-color light transmitted through the color filter form a triangle on the chromatic coordinates having a greater area than the triangle that shows the transmittance characteristics of the color filters.
JP 2003-207770 A discloses a color liquid crystal display device using blue, green and red light emission diodes as a color light source for the backlight together with blue, green and red color filters. The blue light emission diode has a peak wavelength in a range of 430 nm to 480 nm, the green light emission diode has a peak wavelength in a range of 520 nm to 570 nm, and the red light emission diode has a peak wavelength in a range of 620 nm to 660 nm. The color filter has a spectral transmittance of 80% or more at the peak wavelengths of the blue, green and red light emission diodes.
JP 2004-85592 A discloses a liquid crystal display device using a color filter having red, green and blue pixels, wherein the transmittance at a wavelength at which the transmittance in a wavelength range located on the shorter-wavelength side of the wavelength at which the transmittance of the green pixel peaks coincides with the transmittance in a wavelength range located on the longer-wavelength side of the wavelength at which the transmittance of the blue pixel peaks is 10% or less, whereas the transmittance of the green pixel at the peak wavelength is 60% or more and the transmittance of the blue pixel at the peak wavelength is 50% or more.